The present invention pertains to a polarizing beam splitter useful in, among other applications, a projection system. In particular, the polarizing beam splitter combines a prism of relatively high refractive index with a birefringent multi-layer film. The multi-layer film functions as a polarizer and contains at least two different materials, at least one of which exhibits birefringence after uniaxial orientation. The multi-layer film is selected so as to be stable to near UV and blue light.
For projection systems that use reflective liquid crystal display (LCD) imagers, a folded light path where the illuminating light beam and the projected image share the same physical space between a polarizing beam splitter (PBS) and an imager offers a compact design. Most reflective LCD imagers are polarization rotating, i.e., polarized light is either transmitted with its polarization state substantially unmodified for the darkest state or transmitted with its polarization state rotated to provide a desired gray scale. Thus, a polarized light beam is generally used as the input beam. Use of a PBS offers an attractive design because it can function to polarize the input beam and fold the light path.
A PBS is an optical component that splits incident light rays into a first (transmitted) polarization component and a second (reflected) polarization component. One common PBS is the MacNeille polarizer that discriminates between s and p-polarized light as described in U.S. Pat. No. 2,403,731 to MacNeille. In a MacNeille polarizer, the s-polarization is reflected and, over a narrow range of angles near the Brewster angle, the p-polarization is mostly transmitted. The p-component corresponds to light polarized in the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence. The plane of incidence means a plane defined by a reflected light ray and a normal to the reflecting surface.
Some skilled in the art have devised other types of PBS. For example, U.S. Pat. No. 5,912,762 (Li et al.) discloses a thin film polarizing device that may be used in a PBS. The device has first and second light transmissive substrates in the form of prisms and a plurality of thin film layers disposed between the prisms. The thin film layers comprise high refractive index layers and low refractive index layers, the high refractive index layers having one or more different refractive indices and the low refractive index layers having one or more different refractive indices. The light transmissive substrates have a refractive index greater than the refractive index of each of the low refractive index layers. The prisms are shaped so as to allow incident light to strike upon the thin film layers at a plurality of angles greater than or equal to the critical angle (i.e., the angle that generates total internal reflection conditions) for the highest refractive index of the low refractive index layers. Like the MacNeille polarizer, the polarizer in U.S. Pat. No. 5,912,762 discriminates between s and p-polarized light, although in the latter, s-polarized light is transmitted and p-polarized light is reflected.
As another example, WO 00/70386, in FIG. 1, discloses a Cartesian PBS element 50 that includes a multi-layer birefringent film 52 encased in a glass cube 54, and oriented so as to reflect light incident with x-polarization (i.e., approximately s-polarization). See page 11, lines 9 to 11. The notation in WO 00/70386 publication is different in that y-polarization is said to be approximate to s-polarization. For incident rays of light in a large cone angle, the Cartesian PBS has been demonstrated to provide a higher contrast than a PBS that discriminates only on the basis of s-polarization vs. p-polarization.
The technology discussed thus far, although disclosing useful PBS using multi-layer films, may not be well suited for use in a projection system. In such a system, the PBS typically experiences high intensity of light from a wide range of wavelengths possibly for long periods of time. Although the inorganic based multi-layer films of U.S. Pat. No. 2,403,731 and U.S. Pat. No. 5,912,762 may be stable to high intensity blue light, they have deficiencies in angular performance needed in low f-number systems. What is needed to advance the art is a multi-layer film based PBS that has the durability to withstand the light source and simultaneously to provide contrast for incident light in large cone angles so that the resulting image of a projection system, when viewed by an observer, appears bright, sharp, distinct, and possesses crisp colors.
Polarizing beamsplitters can be fabricated from birefringent polymeric multi-layer films, as disclosed in U.S. Pat. No. 5,962,114. Although many polymers exhibit a high transparency to visible light, many have strong absorption peaks in the near ultraviolet (UV) region. As a result, an absorption tail may extend into the visible portion of the spectrum. Although the percentage of absorbed light may be low, the absorbed energy in an intense light beam can result in over-heating of the film leading to thermal induced degradation of the polymer, light induced degradation or both. For some high index polymers, the absorption tail in the blue region is strong enough to impart a yellow color to the film. A key parameter in selecting polymers for a stable multi-layer PBS for high intensity projection systems is the proximity of their absorption edges to the visible spectrum.
The present invention provides a PBS that combines at least one high refractive index (i.e., greater than n=1.60) prism with a birefringent multi-layer film (sometimes referred to as xe2x80x9cmulti-layer filmxe2x80x9d for convenience). The multi-layer film functions as a polarizer. It contains alternating material layers that are stable when exposed to wavelengths associated with near UV light and blue light. These material layers are chosen based on their absorption spectrum within the visible spectrum and on the location of absorption edges in the UV and infrared (IR).
On the UV end of the spectrum, absorption edges for the material layers in the multi-layer film are preferably at least 40 nm less than, more preferably 50 nm less than, most preferably 60 nm less than the shortest wavelength that illuminates the PBS. For color projection displays, blue light below 420 nm can be rejected without substantially affecting the color balance or brightness of the display. Thus in a preferred embodiment, the shortest wavelength that illuminates the PBS is 420 nm. Depending on the light source, the preferred lower wavelength may be shorter, such as 410 nm, or somewhat higher, such as 430 nm. On the IR end of the spectrum, the absorption edges for the material layers in the multi-layer film are preferably at least 40 nm greater than, more preferably 50 nm greater than, most preferably 60 nm greater than the longest wavelength that illuminates the PBS. These considerations may exclude some combinations of materials that can be oriented to produce a high index difference between them in the x (stretched) direction. Practical processing and environmental stability considerations may restrict the set of available materials to those that have a relatively small refractive index difference (i.e., less than 0.15 xcex94nx) between them (in the x direction) after orientation.
In this document, the term xe2x80x9caboutxe2x80x9d is presumed to modify each numerical recitation of a property such as, but not limited to, wavelength, refractive index, ratios, weight percentages, mole percentages. For example, a recitation of 500 nm for wavelength means about 500 nm. The term xe2x80x9cpass axisxe2x80x9d means the optical axis of transmission of the polarizer and corresponds to the y-axis or non-stretch direction of the multi-layer film. The term xe2x80x9cextinction axisxe2x80x9d means the axis of reflection of the polarizer and corresponds to the x-axis or stretch direction of the multi-layer film.
The term xe2x80x9cabsorption edgexe2x80x9d means generally the wavelength at which the polymeric material becomes substantially opaque. A more precise definition is the wavelength at which the transmission, in air at normal incidence, is 10% for a 0.1 mm thick film. Each individual material layer in the multi-layer film has an x-direction, a y-direction, and a z-direction. The x-direction represents the stretch direction (also known as the xe2x80x9ctransverse directionxe2x80x9d or xe2x80x9cTDxe2x80x9d), i.e., the direction in which the film is oriented. The y-direction represents a non-stretch direction (also known as xe2x80x9cmachine directionxe2x80x9d or xe2x80x9cMDxe2x80x9d). The z-direction represents another non-stretch direction and is in the thickness direction of the individual layer.
Although two layers of different refractive indices are typically used in making the multi-layer film, it is within the scope of this invention to use more than two materials. Whereas a two-component multi-layer film presents a square wave index profile to the incident light wave, the optical repeating units in the multi-layer film need not present a square wave. Multiple material layers can be used to construct any periodic modulated index profile along the x direction while having substantially matched indices along the y and z directions. Alternatively, any continuously varying index profile such as, e.g., a rugate filter, can be used to make a birefringent polarizer. The continuously varying index can occur when the materials of a two-component system interdiffuse during processing. Similarly, a continuously varying index exists in cholesteric liquid crystal films.
The multi-layer film of the present invention need not be fabricated by coextrusion and orientation of polymeric materials, but may comprise birefringent organic crystalline layers that are constructed by techniques known in the art such as e.g. epitaxial vacuum deposition.
The material layers of the multi-layer film of the present invention need not have strictly orthogonal optic axes. The orientation of the axes may vary by several degrees from the orthogonal condition, e.g., up to 10xc2x0.
Because of its composition and construction, the inventive birefringent multi-layer film and the resulting PBS exhibit extended durability when exposed to the wide variety of light sources used in a projection system or a display. A typical light source includes a lamp and a reflector. Suitable lamps include xenon, incandescent, laser, light emitting diode (LED), metal halide arc light source, and high-pressure mercury light source. Such light sources can emit light in the blue and near ultraviolet wavelength. It is known that many polymeric-based films can degrade quickly when exposed to such wavelengths.
The inventive multi-layer film, when immersed or embedded in air or low refractive index (i.e., lower than 1.60) prism, exhibits a low contrast ratio (i.e., a contrast ratio of less than 100:1) due to the low difference in the values of the x direction index of refraction for the material layers. The xe2x80x9ccontrast ratioxe2x80x9d means a ratio of two transmission values for light that have the planes of polarization parallel to the two orthogonal axes of the multi-layer film. The contrast ratio will depend on the nature of the beam in addition to the film. For example, the contrast ratio for a light beam in a cone of light distributed over a wide range of angles may be less than for a light beam distributed over a narrow cone of angles.
Advantageously, when immersing or embedding the multi-layer film in a high index (i.e., greater than 1.6 and less than an index that would create total internal reflection condition in the multi-layer film) prism, the contrast ratio increases substantially, on the order of greater than 100:1, preferably greater than 300:1, more preferably greater than 1000:1, when averaged over all rays of the incident cone of light. In one aspect, this advantage means that fewer layers may be required in the multi-layer film to achieve the desired contrast ratio. In general, the lower number of required layers can lead to a less complicated manufacturing process as compared to a similar film having a larger number of required layers. The combination of the multi-layer film embedded in the high index prism yields an improved PBS durable enough to withstand a typical light source used in many projection and display systems and yet provide excellent contrast. The index of the prism is preferably selected such that the highest incidence angles of incoming rays are close to, but not exceeding, the critical angle for total internal reflection (TIR).
Thus, in brief summary, the present invention provides a PBS comprising: (a) a birefringent film having a pass axis, the birefringent film comprising multi-layers of at least a first material layer and a second material layer, each material layers having an absorption edge in the visible spectrum such that in the ultraviolet region, the absorption edge is at least 40 nm less than the shortest wavelength of light that illuminates the polarizing beam splitter and in the infrared region, the absorption edge is at least 40 m greater than the longest wavelength of light that illuminates the polarizing beam splitter; and (b) at least one prism having a refractive index greater than 1.6 but less than a value that would create total internal reflection along the pass axis of the birefringent film. In a preferred embodiment, the shortest wavelength to illuminate the PBS is 420 nm and the longest wavelength is 680 nm. In this embodiment, the preferred absorption edges are at wavelengths less than 380 nm and greater than 720 nm.
Another embodiment of the invention is directed to an optical device, comprising: (a) the PBS described above, a first path being defined through the PBS for light in a first polarization state; and (b) at least one imager disposed to reflect light back to the polarizing beam splitter, portions of light received by the at least one imager being polarization rotated, polarization rotated light propagating along a second path from the imager and through the PBS.
Yet another embodiment of the invention is directed to a projection system that includes a light source to generate light, conditioning optics to condition the light from the light source and an imaging core to impose on image on conditioned light from the conditioning optics to form image light. The imaging core includes a PBS described above and at least one imager.
The present inventive PBS differs from the Cartesian PBS disclosed in WO 00/70386 in that the present invention identifies for the first time (1) the range of refractive index needed for the prism when using relatively low birefringence materials, (2) the wavelength ranges of absorption edges required for a suitably stable polymeric PBS multi-layer film and (3) available materials combinations that exhibit stability when exposed to near UV and blue light.
The present inventive PBS also differs from the PBS disclosed in U.S. Pat. No. 5,912,762. In that patent, it is disclosed that the transparent substrates, i.e., the prisms, have a refractive index greater than the refractive index of each of the low refractive index layers.
With the present invention, on the other hand, the prisms preferably have a refractive index higher than any of the refractive indices of any optical layer in the multi-layer film, but low enough so as not to produce a TIR condition along the pass axis of the birefringent multi-layer film polarizer. The term xe2x80x9coptical layersxe2x80x9d means those layers that participate in the reflection and transmission of the incident light. The internal angles of incidence at the multi-layer interfaces should be sufficiently high so that the interfacial reflectance coefficients at each layer are sufficiently large for x polarized light to produce an extinction ratio that meets required levels of 100:1, preferably 300:1, more preferably 1000:1. The required level of interfacial reflectance for a given wavelength of light can be calculated from the number of layers in the multi-layer film and the layer thickness distribution.
Of related interest is the following U.S. Patent Application, filed concurrently with this application and by the assignee of this invention: xe2x80x9cProjection System Having Low Astigmatism.xe2x80x9d Attorney Docket Number 56696USA5A.002, which is herein incorporated by reference.